Internally shielded teletherapy source

ABSTRACT

AN IMPROVED COBALT-60 TELETHERAPY SOURCE IN WHICH AT LEAST A PORTION OF THE NECESSARY SHIELDING MATERIAL IS INCLUDED WITHIN THE SOURCE CONTAINER. HIGH-ACTIVITY COBALT-60 MATERIAL IS ENCLOSED WITHIN A RING OR CUP-SHAPED SHIELDING MEMBER WITHIN A DOUBLE-WALLED STAINLESS STEEL ENCAPSULATING CONTAINER. THE EXTERIOR DIMENSIONS OF THIS CONTAINER ARE SUCH AS TO FIT WITHIN A SOURCE CAPSULE HOLDER. PREFERABLY, THE INTERNAL SHIELD IN INTIMATE CONTACT WITH THE COBAL-60 HAS A WALL THICKNESS OF FORM ABOUT 0.3 TO ABOUT 0.7 CENTIMETERS AND COMPRISES URANIUM OR TUNGSTEN METAL OR AN ALLOY THEREOF. THIS ARRANGEMENT PERMITS THE USE OFHIGHER ACTIVITY COBALT LOADINGS IN STANDARD TELETHERAPY MACHINES. ALSO, THIS PERMITS MORE EFFECTIVE USE OF THE RADIATION AND MORE ACCURATE AND EFFECTIVE RADIATION THERAPY.

[7 2] Inventors Norma- Fremont; Lawrence E. Wilkinson, Llvermore, Calif.[21] Appl. No. 793,699 [22] Filed Jan. 24,1969 [45] Patented June28,1971 [73] Assignee General Electric Company I 54] INTERNALLY SHIELDEDTELETHERAPY SOURCE 9 Claims, 3 Drawing Figs.

[521 US. Cl. 250/106, 250/84, 250/108 [51] lnt.Cl G2lh5/00 [50] FieldofSearch 250/106 (S), 108, 84

{56] References Cited UNITED STATES PATENTS 2,891,168 6/1959 Goertz eta1. 250/1068 3,145,181 8/1964 Courtois et a1. 250/1065 2,964,628 12/1960Ohmart 250/1065 ABSTRACT: An improved cobalt-60 teletherapy source inwhich at least a portion of the necessary shielding material is includedwithin the source container. High-activity cobalt-60 material isenclosed within a ring or cup-shaped shielding member within adouble-walled stainless steel encapsulating container. The exteriordimensions of this container are such as to fit within a source capsuleholder. Preferably, the internal shield in intimate contact with thecobalt-60 has a wall thickness of from about 0.3 to about 0.7centimeters and comprises uranium or tungsten metal or an alloy thereof.This arrangement permits the use of higher activity cobalt loadings instandard teletherapy machines. Also, this permits more effective use ofthe radiation and more accurate and effective radiation therapy.

PATENTEU JUN28 I97! INVENTORS NORMAN C. HOWARD LAWRENCE E. WILKINSONATTORNEY INTERNAILLY SHIELDED TELETHERAPY SOURCE BACKGROUND OF THEINVENTION Various types of high-energy radiation have been used for sometime in the treatment of certain forms of cancer. Originally, radium wasused in the form of implants and contact molds. Attempts to use radiumin a teletherapy mode, that is, utilizing collimated radiation from aradium source spaced from the patients skin were largely unsuccessfuldue to the low energy of the radium sources which gave a high-surfacedose and a low-depth dose. In addition, because of the low specificactivity of the radium salt, the sources were large and as a result theedges of the radium beam were ill defined. Because of these problems,X-rays became generally preferred in beam therapy. Ideally, the objectof externalbeam therapy is to produce a homogeneous distribution ofabsorbed energy within a prescribed treatment volume and time interval,while producing no energy absorption in the tissues outside this volume.It has been found that the higher the generating voltage, the moretherapeutically useful are the characteristics of an X-ray beam for thetreatment of deepseeded cancer. Thus, X-ray generators with ever highervoltages have been developed.

In the early 1950 s it was found that gamma radiation emitted by thedecay of cobalt-6O is in the very desirable highenergy region and hascertain other advantages over X-radiation. Teletherapy using cobalt-60sources has'a great number of advantages over prior radium and X-rayteletherapy. The attenuation of its high energy (125 Mev.) radiation asit passes through an absorbing medium is not as rapid as that ofradiation of lower energies. As a result, the decrease in dose rate withincreasing depth in tissue is more gradual with cobalt-60 radiation thanwith lower voltage X-radiation or radium-radiation beams.

Cobalt occurs in nature in one isotopic form, with a mass number of 59.Cobalt-60 can readily be produced by neutron irradiation of cobalt-59 ina nuclear reactor. The specific activity in curies of cobalt-60 per gramof the irradiated cobalt is dependent upon the thermal neutron flux towhich the material is subjected in the reactor. Where the neutron fluxis relatively low the specific activity of the cobalt-60 produced willbe relatively low and a large quantity of the material will be requiredto give effective performance in a teletherapy source. For a variety ofreasons, further discussed below, it is highly desirable that thecobalt-60 teletherapy source have relatively high specific activity.

An importantadvantage of cobalt-60 radiation is the fact that the pointof maximum energy absorption for a single beam of cobalt-60 radiation isat a depth of approximately 0.5 centimeters beneath the skin rather thanat the skin surface. Skin reactions are therefore minimal, even whenradical doses are delivered.

At conventional X-ray energies, because of the predominance ofabsorption by the photoelectric process the energy absorbed in bone issignificantly greater than that in soft tissue. At the energy ofcobalt-60 radiation the principle absorption process is that of theCompton effect, for which the energy absorbed per roentgen per gram ofbone and soft tissue is approximately the same. This reduces bone damagewhen the treatment traverses normal bone.

Since all body structures absorb cobalt-60 radiation to approximatelythe same degree, discontinuities in dosage patterns are minimized sothat therapeutic dosimetry is more accurate. Scattered radiation arisingfrom interaction of cobalt- 60 primary radiation with tissue ispredominantly in the forward direction. Because of the small amount ofside-scattered radiation, the falloff of dose at the geometrical edge ofthe gamma-ray beam is rapid, reducing the dose to adjacent sensitivestructures.

Cobalt-60 units are also advantageous over high voltage X- raygenerators because of their simplicity, flexibility and constancy ofoutput. The absence of complicated electric and electronic circuitsreduces maintenance problems and eliminates the need of a highly skilledmaintenance staff. The relative simplicity and small size of the sourcehead and the absence of heavy electrical connections allow a high degreeof flexibility in equipment design. This has resulted in the developmentand production of a wide range rotational cobalt- 60 units.

The radiation source used in the cobalt-60 teletherapy units consists ofa small amount of cobalt-60 in a container which is inserted in theteletherapy head. The usual cobalt-60 beam therapy unit consists of theencapsulated cobalt-60 radioactive source, a source shield or housing, ashutter device to turn the useful beam on and off and a variety ofsupport mechanisms and beam control and adjustment mechanisms. Thesource housing and shutter includes sufficient shielding material, suchas lead, so that the leakage radiation from the source shield willaverage less than 2 milliroentgens per hour at a distance of one meterfrom the source in all directions. While a wide variety of therapy unitshave been designed, a single international standard source holder hasbeen designed which will fit most of these units. This holder canaccommodate a. cylindrical source up to 3 centimeters in diameter by 3centimeters high. The quantity of shielding material included in theunit housing is determined by the radiation leakage limit and theexpected activity of sources to be used therein. Thus, higher outputsources may not be usable in older teletherapy units since they wouldcause leakage radiation exceeding the permissible limits. However,high-output source materials are now becoming available due to improvedability to produce cobalt-60 in high flux level nuclear reactors. Highactivity sources permit the treatment of a deep seated tissue in shorterperiods.

In the international standard source holder, the maximum source diameteris 3 centimeters. Of course, smaller diameter sources may be used byinserting a spacer ring (or shim) between the outer wall of the sourceand the inner wall of the holder. In general, it is desirable to havethe smallest possible diameter source. As the source diameter approachesa true point source and a diaphragm is used between the source and theobject receiving the radiation, radiation will fall off sharply at theedge of the field. However, where the source has a large diameter, alarge penumbra will result; that is, the radiation falloff outside thefield will be more gradual. This results from the fact that radiationcoming from one side of the source is limited by the diaphragm aperturein a different way than that emerging from the opposite side. Anextensive penumbra is undesirable since it will result in theirradiation of healthy and possibly sensitive tissues adjacent theportion being treated.

Thus, there is a continuing need for improved cobalt-60 teletherapysource configurations which permit the use of higher specific activitysource materials without exceeding permissible leakage radiation andwhich improve the accuracy and precision of the therapeutic radiation.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide a teletherapy source overcoming the above-noted problems.

Another object of this invention is to provide a cobalt-60 teletherapysource permitting the use of higher output sources in existing machines.

Still another object of this invention is to provide a teletherapysource permitting a more precise radiation beam.

Still another object of this invention is to provide a teletherapysource of improved convenience, reliability and safety.

The above objects and others are accomplished in accordance with thisinvention by providing a cobalt-60 teletherapy source in which thecobalt-60 source material is at least partially surrounded by and inintimate contact with a quantity of shield material which is in turndoubly encapsulated in stainless steel containers, the outside dimensionof which fits within the international standard teletherapy sourceholder. Preferably, the cobalt- 60 material is in the form of smallpellets in a generally cylindrical array within a ring, ringand-cap, orcup-shaped shield member. The radial walls of said shield memberpreferably have a thickness of from about 0.3 to about 0.7 centimeters.This thickness range is preferred since much thinner walls tend todecrease the shielding capability to the point where higher outputcobalt- 60 sources cannot be safely used while much thicker walls woulddecrease the diameter of the core to the point where unachievably highspecific activity would be required in the source material. The internalshield member and source material are then doubly encapsulated instainless steel cans having a thickness of about 0.65 millimeters.Stainless steel is preferred for the encapsulating cans because of itsstrength, corrosion resistance and ability to be easily closed with highintegrity welds. it is undesirable to use a high-density material forthe encapsulating container, since this would also attenuate theradiation in the treatment beam.

While any material having a density greater than about grams per cubiccentimeter would be useful in the internal shield member, unalloyed andunclad uranium metal or a suitable tungsten alloy are preferred for usein the internal shield. Uranium performs extremely well as the internalshield. Preferably, the uranium is unalloyed and unclad since anyalloying or cladding materials would have lower densities and woulddecrease the shielding ability of the internal shield member.Applicants'have found that uranium in the unclad state may be used inthe configuration of this invention since the internal shield materialis entirely encapsulated. Uranium metal is difficult to use in anexternal shield in air since it is easily oxidized and then expands involume, Thus, an external uranium shield may expand to the point whereit prevents disassembly of close-fitting components. Tungsten is alsopreferred for the internal shield material because of its high densityand its availability at reasonable cost.

It has been found that the addition of the internal shield member madeup of very effective shielding material in intimate contact with thesource material is much more effective than the equivalent quantity ofshielding material spaced further from the source material. The mass ofshielding material to give an equivalent shield thickness is smallerwhere the shield is contained within the source material encapsulatingcans than where the shield material is in the form of a ring outside theencapsulating cans. Also, the larger external shield ring would permit agreater quantity of radiation to leak out of the ends of the ring. Wherethe shielding is internal, the end leakage beams are significantlynarrower.

The attenuation of gamma radiation through matter is expotential in thatfor a particular radiation energy successive equal thicknesses of amaterial will reduce the radiation intensity by equal fractions. Forexample, a 1.2 centimeter thickness of lead attenuates primary cobalt-60 radiation by a factor of 2, while an additional 1.2 centimeter willreduce the original exposure by a total reduction factor of 4. Thus, ifit is required that the exposure dose rate be reduced by a factor of1.000 in order to reduce the radiation leakage adjacent a therapy unitto permissible limits, then, since l024=2power, a thickness of 12centimeters of lead will be necessary for primary beam protection. Thethickness of an absorber which attenuates a beam of radiation byone-half is called the halfvalue thickness.For primary cobalt- 60radiation the halfvalue thickness of uranium is 0.566 centimeters and oftungsten is 0.69 centimeters. Thus, the inclusion of a 0.3 to 0.7centimeter internal shield of uranium in a source capsule will increasethe effective shielding of the overall unit by a factor of about 0.53 to1.25. For tungsten this factor is about 0.43 to 1.02. Thus, with theteletherapy source of this invention, the permissible activity level ofthe source in a conventional teletherapy machine will be increased byfrom about 40 percent to about 125 percent.

As the use of the internal shield permits the use of higher activitymaterials, the source may be made narrow er in order to approach morenearly to the ideal point source. This will greatly decrease thepenumbra effect and substantially decrease the undesired irradiation ofsensitive structures adjacent to the matter being irradiated.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention and ofpreferred embodiments thereof will be further understood upon referenceto the drawing, wherein:

FIG. 1 shows an isometric view, partly cut away, of a teletherapy sourceaccording to this invention positioned in an international standardsource holder;

FIG. 2 shows an isometric view, partially cut away, of a secondembodiment of the teletherapy source of this invention positioned in aninternational standard source holder; and

FIG. 3 shows a schematic representation of a typical cobalt- 60teletherapy machine including the teletherapy source of this invention.

DETAILED DESCRIPTION OF THE lNVENTION Referring now to FIG. 1 there isseen a teletherapy source generally designated 10 enclosed within aninternational standard source holder generally designated 11. Thedimensions of source holder 11 have been made standard by internationalagreement. Source holder 11 carries external threads 12 to permit thesource holder to be threaded into a teletherapy machine. Source holder11 is loaded by inserting thecylindrical source 10 into the standarddiameter bore of source holder 11. An annular inwardly projecting lip 13prevents source 10 passing through the bore. A retaining ring 14 engagedin an annular slot in the bore of source holder 11 retains source 10 inposition.

As seen in FIG. 1, source 10 consists of a quantity of cobalt- 60 sourcematerial 15 within a ring-shaped internal shield member 16 both of whichare doubly encapsulated within two concentric stainless steel cans l7and 1.8. The end members of cans 16 and 17 are welded in place toprevent leakage of radioactive material.

This configuration permits the use of a high-activity cobalt- 60 sourcematerial in a standard teletherapy machine since additional shieldingmaterial is provided in the most effective location. Also, the cylinderof cobalt-60 source material is rela tively narrow thus decreasing theexit beam diameter and, therefore, the undesirable penumbra efiect.Since the internal shielding material 16 is entirely enclosed withindouble-encapsulating cans 17 and 18, materials such as uranium metalwhich tend to swell when exposed to the atmosphere may be used withoutinconvenience. If an attempt were made to use a ring of uranium metalbetween a narrow source cylinder and the internal wall of source holder11, over a period of time the uranium would swell making removal of theshielding and source material from the source holder difficult, if notimpossible. Also, the use of shielding material between the externalwall of the source and the internal wall of the source holder would bedifficult since the international standard source holder has noprovision for retaining such an assembly in position. Further, a greateramount of the relatively expensive shielding material would be requiredto give the desired thickness if the shielding material were interposedbetween the outer wall of the source and the inner wall of the sourceholder instead of within the source cans as provided in the teletherapysource of this invention.

FIG. 2 shows a second embodiment of the teletherapy source of thisinvention. As in FIG. 1, the source 10 is retained between a lip 13 anda retaining ring 14 within the international standard source holder 1 1.However, in this instance the internal shield member 16' has a generallycylindrical ringand-cap or cup-shaped configurationrather than theringshape shown in FIG. 1. The configuration shown in FIG. 2 will bepreferred where source activity is very high. The internal shield member16 provides additional shielding both at the sides of source material 15and at the back of the source. Of course, shielding in the beam portregion would be undesirable since this would interfere with the use ofthe source in irradiating objects. Also, where the specific activity ofthe source material is very high, and the diameter of the cylinder ofsource material is relatively narrow, the portion of source material atthe end of the source material cylinder away from the beam port willtend to be self-shielded by the source material closer to the beam port.Therefore, adding additional shielding material in the form of a cap orcup base at the back of the source in place of a portion of sourcematerial will not significantly decrease the effectiveness of a veryhigh-activity source material while significantly increasing the shieldeffectiveness in this direction.

FIG. 3 shows a schematic representation of a typical teletherapy machinein which the teletherapy source of this invention is especially useful.The teletherapy head generally designated is positioned above the object21 to be irradiated on any conventional supporting structure (notshown). A generally cylindrical shutter drum 22 is positioned in ahorizontal cylindrical opening throughhead 20. Drum 22 is rotatable by adrive means 23. Adjacent one edge of drum 22 is a horizontal openingwithin which source drawer 24 is slidably inserted. Drawer 24! is heldin place by a removable end cover 25. The teletherapy source in sourceholder 11 may be directly inserted into a cavity in drawer 24, or thecavity may be sized and threaded to receive teletherapy source in sourceholder 11 in source drawer 24. Drive means 23 is capable of rotatingdrum 22 from an of! position as shown, in solid lines in FIG. 3, inwhich the source is surrounded by shielding material to an on position(shown in broken lines) in which the source in source holder 11 isadjacent a port 26 which permits radiation ofobject 21.

Collimating blocks 26 are provided to adjust the width of theirradiation beam, which is indicated by lines 27. As can be seen fromFIG. 3, where the source in source holder 11 is a point source, line 17will outline a sharp, distinct area on object 21. Where the source insource holder 11 has an appreciable source diameter, it is apparent thatthe edges of the area outlined by lines 27 will be vague and indistinct,thus making the treatment of a distinct zone on object 211 difficult.

As is apparent in H6. 3, when the source is in the off" position, alarge mass of shielding material 28 has been installed in head 20adjacent the open or beam port end of the source holder. However, sincethe physical dimensions of drawer 24 and of drum 22 behind source holder11 are fixed, it is difficult to add shielding at the sides and/or backof the source material where it is desired to use higher activitysources than those for which the source head was originally designed.Thus, Applicants internal shielding concept will permit a substantialincrease in the capabilities of commercial teletherapy machines.

Although specific components and proportions have been described above,other materials as indicated above, may be used with similar resultswhere suitable. In addition,, other materials may be added to the sourcematerial, shielding material, etc. to enhance or otherwise modify theirproperties.

Other modifications and applications of the present invention will occurto those skilled in the art upon reading the present disclosure. Theseare intended to be included within the scope of this invention.

We claim:

1. A teletherapy source comprising:

a. a quantity of high specific activity cobalt-60 teletherapy sourcematerial in generally cylindrical configuration;

b. a quantity of a shielding material having a density greater thanabout 10 grams per cubic centimeter at least partially surrounding andin intimate contact with said source material;

c. a first closed stainless steel container surrounding saidv sourcematerial and shielding material; and

d. a second closed stainless steel container surrounding said firststainless steel container.

2. The teletherapy source according to claim 1 wherein said cobalt- 60source material has a specific activity of from about 300 to about 500curies per gram.

3. The teletherapy source according to claim 1 wherein said shieldingmaterial is generally ring shaped and has a wall thickness of from about0.3 to about 0.7 centimeters.

4. The teletherapy source according to claim 1 wherein said shieldingmaterial is substantially cup shaped and has a substantially uniformthickness of from about 0.3 to about 0.7 centimeters.

5. The teletherapy source according to claim 1 wherein each of saidstainless steel containers has a wall thickness of about 0.65millimeters.

6. The teletherapy source according to claim 1 wherein said shieldingmaterial is selected from the group consisting of unalloyed, uncladuranium metal and tungsten alloys.

7. In a cobalt- 6O teletherapy source assembly comprising an annularinternational standard teletherapy source holder, secured within saidholder a source capsule comprising a quantity of cobalt-60 teletherapysource material doubly encapsulated within stainless steel containers;the improvement wherein a quantity of a shielding material having adensity greater than about 10 grams per cubic centimeter is providedbetween, said cobalt-60 source material and said encapsulatingcontainers, said shielding material at least partially surrounding andin intimate contact with said source material.

8. The assembly according to claim 7 wherein said cobalt- 60 sourcematerial has a generally cylindrical configuration and said shieldingmaterial is in the form of a ring-shaped member surrounding and inintimate contact with said source material and in contact with the innerwall of the smaller of said encapsulating containers, said shieldingmaterial being selected from the group consisting of tungsten alloys andunalloyed, unclad uranium metal, said ring-shaped member having a wallthickness of from about 0.3 to about 0.7 centimeters.

9. The assembly according to claim 7 wherein said shielding materialisgenerally cylindrical with an axial opening in one end filled with saidcobalt-60 source material, said shielding being material selected fromthe group consisting of unclad, unalloyed uranium metal and tungstenalloys, said shielding material having a substantially uniform wallthickness in the range of from about 0.3 to about 0.7 centimeters.

